RFR: 827: Copyright year differences can be regarded as a clean backport [v3]

[Zhao Song]

> As many users requested, when determining whether a backport is clean, our bot should ignore copyright year difference.

Zhao Song has updated the pull request incrementally with two additional commits since the last revision:

 - fix whitespace issue
 - add unit test

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1556/files
  - new: https://git.openjdk.org/skara/pull/1556/files/0af301eb..bff1ad28

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1556&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1556&range=01-02

  Stats: 126 lines in 2 files changed: 116 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1556.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1556/head:pull/1556

PR: https://git.openjdk.org/skara/pull/1556